A tachyarrhythmia generally refers to a heart rate that is faster than 100 beats per minute. Tachyarrhythmias can be either physiologic such as sinus tachycardia occurring during exercise or pathologic as during atrial or ventricular tachycardia which can occur when irritable cells in the heart muscle or heart's electrical conduction system start to fire faster than the heart's normal rhythm. Tachyarrhythmias can start in either the upper heart chambers (atria) or lower heart chambers (ventricles). An atrial tachyarrhythmia starts in the atria and is generally classified as being atrial tachycardia (AT), atrial flutter, or atrial fibrillation (AF). A ventricular tachyarrhythmia starts in the ventricles and is generally classified as being either a ventricular tachycardia (VT) or ventricular fibrillation (VF). VT and VF can diminish the ability of the ventricles to pump blood to the body, which can lead to a quick depletion of oxygen to the body, a potentially life-threatening condition.
Implantable medical devices (IMDs) are well known in the medical device field. The prior art is replete with IMDs that are designed to monitor heart activity, detect tachyarrhythmia, provide pacing therapy, and/or provide defibrillation therapy. One type of IMD is known as an implantable cardioverter-defibrillator (ICD). An ICD is a device that is implanted in patients who are at high-risk of sudden cardiac death to provide prompt defibrillation to patients who experience VT/VF episodes. The ICD monitors the rate and rhythm of the heart and can deliver therapies, including defibrillation, when the rhythm is determined to be VT or VF.
Early ICDs were single chamber, and discriminated heart rhythms on the basis of heart rate alone. Slower tachycardias (e.g., <150 beats per minute) were considered to be supraventricular tachycardia (SVT) and received no therapy. Tachycardias between 150 and 190 beats per minute, in a typical usage, were classified as ventricular tachycardias and received rate-appropriate therapy. Tachycardias faster than 190 beats per minute were classified as VF and received defibrillation therapy. Heart rates of SVTs and VTs overlap as do heart rates of VT and VF. Therefore, it was common for therapy to be delivered when none was needed when using rate-only detection schemes. More recently, new criteria have been developed to improve discrimination between SVTs and VTs/VFs to help reduce the frequency of inappropriate therapy. These criteria included, for example, rate of onset (e.g., how quickly the heart increases), rate stability (e.g., how consistent the ventricular intervals are) and electrogram morphology (e.g., how similar the QRS complex is to SVT or VT).
Dual chamber ICDs were introduced that monitor the rate and pattern of both atrial and ventricular activity. These ICDs use sophisticated algorithms to determine whether the rhythm is SVT, requiring no therapy, or VT/VF, requiring therapy. One example of a dual chamber ICD is described in U.S. Pat. No. 7,031,771, issued to Brown et al. on Apr. 18, 2006, and is incorporated herein by reference in its entirety.
In addition, numerous detection and classification systems have been proposed. Many ICDs implement detection and classification strategies that identify heart events, event intervals or event rates as they occur as being indicative of the likelihood of the occurrence of specific types of arrhythmias, with each type of arrhythmia having a preset group of criteria which must be met precedent to detection or classification. As events progress, the criteria for identifying the various arrhythmias are all monitored simultaneously, with the first set of criteria to be met resulting in detection and diagnosis of the arrhythmia.
It is desirable to provide methodologies for identifying and distinguishing various types tachyarrhythmia from one another and for providing appropriate therapies to treat the identified tachyarrhythmia.